1. Field of the Invention
The present invention relates to a picture data reproducing system for use with a video on-demand system that provides the user with picture data and sound data through a network corresponding to a request received from a user, and in particular to a picture reproduction position detecting system for detecting the start position of a reproduced picture.
2. Description of the Related Art
In a video-on-demand system for providing a user with picture data and sound data through a network corresponding to a request received from the user, it is necessary to accomplish an operability and a response that are the same as those with which the user reproduces picture data using a video cassette recorder, without being conscious of a network and a center that provide the user with the data.
FIG. 1 is a block diagram showing a structure of a conventional video-on-demand system. In FIG. 1, the system is composed of a video-on-demand center 1 that provides a user with picture/sound data, a network 2, a television set 3 with which the user side reproduces the picture/sound data, a set top unit (STU) 5 that sends a request of the user from a remote controller 4 through the television set 3 to the center 1 corresponding to a predetermined protocol, a personal computer 6 used by another user as a substitute for the television set 3, and so forth.
When a video server in the video-on-demand center 1 sends picture/sound data requested by the user, the video server should quickly detect the stored position and the playback start position of the requested picture data. As requests received from the user, there are a playback request, a pause request, a rewind request, a fast-forward request, and so forth. The video server should quickly and accurately detect the picture/sound sending position or the playback resume position corresponding to such requests and send the requested data.
FIG. 2 is a schematic diagram showing an example of a format of digital picture data and sound data stored in the video-on-demand center 1. Conventionally, picture data is converted into digital data, compressed, and then stored. Digital picture data is compressed and stored on a disk of the server. On the disk of the server, the digital picture data is stored at a position corresponding to sound data. In addition, time data necessary for synchronizing the picture data with the sound data, playback time data, and so forth are added to the digital picture data.
In FIG. 2, "data type" in the header information represents an identification of picture data or sound data and playback speed. "Compression code type" represents an encoding system to be used. (In the embodiment of the present invention, the MPEG (Moving Picture Expert Group), as an encoding system that allows video data to be effectively stored in a digital data storing medium, is used.) "Playback start relative time" represents a relative time period after picture data is played back until data including the header data is played back. The header data also includes the "amount of data".
In conventional video-on-demand systems, there are two systems for detecting a picture data playback position corresponding to a request received from the user. In the first system, the playback start relative time in the header data shown in FIG. 2 is read. The playback start position is then detected corresponding to the relative time.
In the second system, all the header data is read beforehand. A table having the relation of the playback start relative time of the header data and the picture data storing positions on the disk is created. In the second system, the table is searched for the playback start position.
However, in the first method, whenever a request is received from the user, the disk is accessed for each header data so as to read the playback start relative time included in each header. Since digital picture data is compressed and stored, the playback time varies for each digital picture data. In this system, the amount of data to be processed becomes huge. For example, when many users send their requests to the video-on-demand center, it cannot handle them all at the same time.
In the second system, since the table that has been created beforehand is searched for the playback start position of the requested picture data, the performance is higher than that of the first system. However, in the second system, after the table is searched with a pointer of time, the disk should be accessed. Thus, it is also difficult to handle requests received from many users at the same time.